Well, I know I didn't give much information, but I think I might have found the problem.
I first though that it could be a motherboard problem (I don't know, maybe some incompatibilities with some HDD that I didn't know about), but seemed unlikely because everything else, SSD included, works fine. So I tried to google my HDD model with "bug" or "problem".
My bad luck with hard drives is legendary. Really, the only two (1, 2) hard drives that I bought in the last years, all of them had firmware problems. Well, even the SSD () have one, fixed partially by firmware update.
So, I found an old SATA PCI controller, and tried connecting the HDD to it and... still the same problem. Time to search something about this hard drive.
I found this thread in T's H and they solution they found is to change the power settings to "never stop".
Can you please be specific exactly what setting in the xw6600 BIOS you are talking about. I find no such setting anywhere, and am wondering how you actually set your xw6600 up. Or, is this just a reference to another type of computer and BIOS?